Cloud computing is as confusing as it is promising. For each so-called sure-fire proclamation (all email will soon be in the cloud; a down economy will accelerate cloud computing), there are many more questions.

What are the benefits of a private cloud versus public cloud? How do you manage identities in the cloud? Should the collision of cloud computing and smartphones scare the pants off of our IT staff?

The ten cloud computing leaders listed here hope to help the industry make sense of this trend. Of course, whoever comes out on top will reap massive rewards, influencing how the cloud takes shape in the coming years.

The companies below were chosen based on such relatively objective measures as customer traction and length of experience with the cloud model, as well as plenty of subjective consideration, including ability to innovate, how well cloud computing ties in with the rest of the companys product portfolio and ease of use.

Please note: this is a first in a series of articles. In the next installment well cover cloud computing leaders such as IBM, AT&T, Elastra, Joyent, Cisco, OpSource, Enomaly and several other early cloud innovators. Stay tuned.

Why they're a leader today: Amazon spent years optimizing the back-end infrastructure of its retail service, building it into what is widely recognized as one of the most reliable, scalable, and cost-efficient web infrastructures.

[Amazon Web Services (AWS)] gives any software developer the keys to this infrastructure, which they can use to build and grow any business, an Amazon spokesperson wrote me. This makes it possible for any business to reach the scale of major internet players like Amazon.com, but without the expensive price tag they would have to pay to build and maintain such an . . . infrastructure.

Why they could be on top in years to come: Were I a betting man, Id wager that Amazon will be leading the cloud space for years to come. Why is this such a safe bet? Four factors stand out: track record, cost, flexibility and pace of innovation.

Amazon entered the cloud game early and has a ton of goodwill built up, while also having one of the few established cloud brand names with AWS and Amazon EC2. As the AWS technology platform has become more tested and proven, Amazon has been able to squeeze increasing operational efficiency out of the platform, allowing them to drive down prices.

Were very comfortable with running high volume, low margin businesses, which is very different than traditional IT vendors, Amazons spokesperson noted.

Flexibility, which is one of the clouds great selling points, is also a trait that Amazon believes distinguishes it from other cloud providers. Developers are free to use whatever program languages and operating systems they please, while also having the flexibility to pick and choose services without getting lured into vendor-lock.

Finally, Amazons pace of innovation has been impressive. Their early forays into the cloud, while cutting-edge at the time, would seem outdated today. However, theres nothing outdated about Amazons current lineup of cloud services, and Amazon has shown no signs of slowing down.

This month, for instance, Amazon improved the streaming media capabilities of CloudFront by adding the ability to get detailed activity records on every stream. For businesses, information on who is watching what and when, as well as when they get bored and abandon the stream, is incredibly valuable.

Key Executive: Andy Jassy, SVP, Amazon Web Services and Amazon Infrastructure, joined Amazon in 1997. During his tenure at the company, Jassy authored the business plan for Amazons entry into the music business and served as both its Director of Product Management and GM; started and built the CRM management team; and most recently, started and continues to lead the Amazon Web Services business.

Customers: We have hundreds of thousands of registered developers to date and this includes a broad spectrum of companies around the world, ranging from smaller companies like Playfish to larger companies such as Eli Lily, Netflix, National Geographic, The New York Times Company, NASDAQ, ESPN and NASA, Amazons spokesperson said.

Why they're a leader today: The evolution of SaaS to cloud computing is a natural one. After becoming a force in the CRM market, Salesforce.com rolled out the cloud-based Force.com platform in 2005.

The company claims that its Sales Cloud 2 currently owns 50 percent of the SFA market share. Service Cloud 2 pushes Salesforce.coms cloud reach into customer-service contact centers, giving customers the ability to merge various cloud computing platforms with traditional contact-center channels.

According to a company spokesperson, Salesforce.com became the first enterprise cloud computing company to achieve $1 billion in revenue in 2009.

Why they could be on top in years to come: Salesforce.com has been in the space a long time, long enough to have been called everything from an ASP to a SaaS company to, now, a cloud provider.

The companys latest cloud innovation, Chatter, gives business apps many of the same features as social-networking applications. As the Salesforce credo explains: We are in the era of Cloud 2, where social networking use has surpassed e-mail; Facebook and YouTube use have outpaced search and new mobile devices like the iPad are creating entirely new ways to interact with information.

Chatter leverages profiles, status updates and real-time feeds to enable deep collaboration. Users can follow people, applications and data in a similar manner to how they would engage with friends on Facebook or follow people on Twitter.

Key Executive: Chairman and CEO Marc Benioff is a fervent cloud advocate. His recent book, Behind the Cloud, tells the story of how Salesforce.com went from an idea to a billion-dollar company.